published_or_final_version / Physics / Master / Master of Philosophy

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

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Cyclotron resonance has been measured in GaAs and related compounds through the use of a new experimental technique developed for the study of very pure semiconductors called optically detected cyclotron resonance (ODCR). ODCR differs from other forms of magnetospectroscopy in that the intensity of luminescence excited by a visible laser is monitored rather than the direct absorption of far-infrared radiation. The ODCR technique is initially used on an exceptionally pure sample of GaAs and resolves impurity transitions and central cell effects. An accurate measure of the electron effective mass including band nonparabolicity is made. The ODCR signal represents an interaction between the donor bound electron states and the donor bound exciton states. Standard cyclotron resonance measurements on high mobility GaAs/Ga₁-_xAl_xAs heterojunctions are performed with tilted magnetic fields. Resonant coupling between the Landau levels and electric subbands gives rise to a splitting of the cyclotron resonance lineshape which can be used to determine the subband energy spacings. This allows for a direct determination of the shape of the confinement potential which changes dramatically under different illumination conditions. A dilution refrigerator is modified in order to measure the cyclotron resonance (CR) to 100 mK of the low density, two dimensional electron system in a heterojunction. Anomalies in the CR spectrum are explained in terms of an interacting electron system composed of carriers in the two spin states of the lowest Landau level. Experimental results are presented in terms of a recent theory offered for cyclotron resonance. A series of undoped GaAs quantum wells is studied with ODCR. The conduction band mass was measured for different well widths. An offset is observed in the cyclotron resonance energy which is strongly dependent on the well width. ODCR is measured on In_0.05Ga_0.95As/GaAs superlattices using both the Faraday and Voigt magnetic field orientations. Cyclotron resonance in the Voigt geometry reveals a band structure in the growth direction. The impurity transition in the Voigt configuration shifts dramatically, moving from the bulk 1s-2p + to close to the bulk free electron field.

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This thesis contains a theoretical and experimental investigation of semiconductor quantum dots (artificial atoms). The first part presents a numerical study of spin effects in GaAs/AlAs modulation-doped quantum dots containing 0 to 50 electrons. A theoretical model is developed to calculate confinement potentials and ground-state electron density distributions using the Kohn-Sham local spin-density approximation. Spin polarization, defined as the difference between the up- and down-spin electron densities, is predicted to occur spontaneously in symmetric quantum dots and in quantum point contacts in the lowdensity regime as a result of electron exchange interactions. This spontaneous magnetization can be controlled by an applied gate voltage, which opens up applications in spintronics and provides a possible explanation for the non-integer quantization of the quantum point contact conductance. The second part describes experimental techniques to investigate photon-exciton coupling in InAs/GaAs self-assembled quantum dots. Two experiments on resonant excitation of a single quantum dot are proposed, whereby the quantum-dot emission is distinguished from resonant pump light by either photon bunching of collected photons with reference photons, or Michelson interferometry. The feasibility study of the proposed experiments shows that the photon-exciton coupling efficiency must be dramatically increased by putting the quantum dot inside an optical microcavity. Novel types of high-quality, low mode-volume semiconductor microcavities containing quantum dots are designed, fabricated, and studied on a newly built setup. We present the first results of photoluminescence studies of InAs quantum dots inside both GaAs single-defect square-lattice photonic-crystal slabs and GaAs/AlAs micropillars, and InAs artificial molecules formed by vertically coupled strain-assisted quantum dots. The results indicate the potential of these nanostructures for implementing resonant transfer of quantum information, developing quantum repeaters and entangled-photon sources, and studying QED effects in the strong-coupling regime.

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